This invention relates generally to media handling systems for inkjet printing devices, and more particularly to a media handling system which is capable of achieving small bottom margins.
Known inkjet printing devices which use single sheet or cut sheet media have a limited bottom margin capability. One of the smallest bottom margins achievable is approximately 11.7 mm by the Hewlett Packard 800 series DeskJet.TM. printers. Many applications could take advantage of a smaller bottom margin, if available on single sheet and cut sheet inkjet printers. Continuous form inkjet products are able to achieve smaller bottom margins because a current page is attached to a subsequent page during printing. The pages are detached after printing.
Other inkjet printing concerns which impact the bottom margin limitation are the need for accurate dot placement and the need to account for the effects of wet ink printing. Both of these concerns impact a larger portion of the media sheet than simply the immediate area being printed at any given time. Media handling is one function controlled to achieve accurate printing and wet ink control. In the series 800 DeskJet.TM. printers, for example, pinch rollers keep the media sheet in contact with a drive roller as the media sheet is fed through a print zone adjacent to a printhead. The pinch rollers prevent media slippage and allow for accurate dot placement. Cockle control devices such as ribbed devices place a known bend pattern in the paper downstream from the print zone which limits cockle growth in the print zone. The pinch rollers isolate the cockled area from a flat media sheet area in the print zone.
FIG. 1 shows a conventional inkjet printing apparatus 10 including an inkjet pen 12 having a printhead 14. The printhead 14 includes a plurality of inkjet nozzles which eject ink onto a media sheet 16 during printing. The media sheet is moved along a media path in a direction 17 by one or more rollers, including a drive roller 18. A pinch roller 20 presses the media sheet to the drive roller 18. A platen 22 supports the media sheet as the media sheet 16 is moved through a print zone 24. Typically, the print zone is located close to the pinch roller's line of contact with the media sheet, but further along the media sheet path than the pinch roller's line of contact. More specifically, the print zone 24 is located adjacent to the printhead nozzles between the printhead 14 and the platen 22. The relative location of the pinch rollers relative to the print zone determines how small the bottom margin can be. Once a trailing edge 26 of the media sheet 16 passes beyond the pinch roller 20, there is nothing securing the media sheet as the trailing edge 26 advances through the print zone 24. Accordingly, printing on the media sheet after contact is lost with the pinch roller 20 is subject to inaccuracies. The manufacturer avoids these inaccuracies by making the minimum bottom margin large enough that the media sheet is still in contact with the pinch roller at the bottom margin. Typically the distance from the nearest edge of the print zone to the pinch roller line of contact equals the minimum bottom page margin achievable for an inkjet print apparatus. Referring to FIG. 1, the minimum bottom page margin for the media sheet 16 is limited by the distance d.sub.p from the pinch roller 20 line of contact to the nozzle area of the printhead 14.
In a page wide array inkjet printhead the nozzle rows are oriented 90 degrees about the typical scanning printhead nozzle row orientation. In the FIG. 1 scanning inkjet pen 12 the nozzle rows are oriented in a direction across the drawing sheet from left to right in the areas marked for the printhead 14. FIG. 2 shows a page wide array configuration. Inkjet pen 12' includes a pagewide array printhead 14' having a pair of rows 15, 17 for each color (e.g., YMCK--yellow, magenta, cyan and black). The rows 15, 17 are oriented to extend into the page of the drawing sheet.
Referring to FIG. 2, the media sheet 16 is moved along a media path in the direction 17 by one or more rollers, including a drive roller 18. A pinch roller 20 presses the media sheet to the drive roller 18. A platen 22 supports the media sheet as the media sheet 16 is moved through a print zone 24. Once a trailing edge 26 of the media sheet 16 passes beyond the pinch roller 20, there is nothing securing the media sheet as the trailing edge 26 advances through the print zone 24. Accordingly, the minimum bottom margin is made large enough that the media sheet is still in contact with the pinch roller. With the nozzles oriented in the direction parallel to the length of the pinch rollers and drive rollers, the minimum bottom margin is limited by the first row of nozzles located farthest from the pinch roller 20 (row 17 of color K in FIG. 2). Referring to FIG. 2, the minimum bottom page margin for the media sheet 16 is limited by the distance d.sub.p' from the pinch roller 20 line of contact to the nozzle area of the printhead 14'. For a YMCK printhead, the minimum bottom margin would be greater than 1 inch.
One way of reducing the minimum bottom margin is to place the pinch roller 20 closer to the print zone 24. There is a limit, however, to how close the pinch roller line of media sheet contact can be to the print zone. Another scheme is to make the pinch roller diameter smaller, so that the distance between the print zone and pinch roller can be shorter. However, media advance accuracy suffers as the pinch roller becomes too small.
The pinch roller also serves to provide a reverse bowing which reduces cockle growth from the wet ink printing. Cockle growth refers to the buckling or ridges in a media sheet due to the presence of wet ink soaking into the media sheet. As the pinch roller becomes too small the reverse bow desired for limiting cockle growth becomes difficult to maintain. Accordingly, there is a need for a method and apparatus for allows for smaller bottom margins than the distance between pinch roller and print zone.